PNNL

Abstract

Despite a seemingly simple appearance, cyclobutanetetraone (C4O4) has four low-lying electronic states. Determining the energetic ordering of these states and the ground state of C4O4-theoretically has been proven to be considerably challenging and remains largely unresolved to date. Here we report a low-temperature negative ion photoelectron spectroscopic approach. A well structured spectrum with evenly spaced features was obtained at 193 nm due to excitation of the ring breathing mode of the C4O4 neutral, whereas each 193-nm feature was observed to further split into a three-peak manifold at 266 nm assigned due to three electronic transitions from the ground state of the anion to the ground and two low-lying excited states of the neutral. Combined with recent theoretical studies and our own Franck-Condon factors simulations, the ground state of C4O4-, as well as the ground and two low-lying excited states of C4O4 are determined to be 2A2u, and 3B2u, 1A1g (8p), 1B2u, respectively. The frequency of the ring breathing mode (1810 ± 20 cm-1), the electron affinity (3.475 ± 0.005 eV), and the term values of 1A1g (8p) (6.27 ± 0.5 kJ/mol) and 1B2u (13.50 ± 0.5 kJ/mol) are also directly obtained from the experiments